A Resistor-Capacitor Highpass Filter is a simple analog filter that allows high frequencies to pass but attenuates lower frequencies. The cutoff frequency of the filter is determined by \(F_c=\frac{1}{2\pi RC}\)..
An RC highpass filter effectively forms a frequency dependent voltage divider. At low frequencies, the capacitor acts as a very high resistance, so the signal is attenuated a lot. At higher frequencies, the capacitor has less resistance, so the signal is attenuated less.